The present invention relates to a heat transfer device, a method of transferring heat and a method of manufacture of a heat transfer device. More particularly, the present invention relates to a device and method for transferring heat from a heat source to a heat sink through thermally conductive fibers in a composite material.
The conduction of heat away from heat generating components has long been a problem, particularly where the components are adversely affected by an increase in temperature. This problem is often exacerbated in electronic systems where such components are frequently grouped in close proximity on mounting boards. (The term "component(s)" refers to both a single component and component assemblies of more than one component.) One solution to this problem has been to use the mounting boards to conduct heat away from the components to a remote heat sink. In some applications, such as in airborne electronic systems, it is particularly desirable to use lightweight mounting boards and composite materials have been developed that are suitable for use in lightweight boards for airborne systems.
Composite boards of materials such as carbon pitch fiber may comprise a core of binder material and fibers that are highly thermally conductive. Typically, the fibers in the core are arrayed in rows in parallel layers and held together by the binder. The core may contain a significant amount of binder by volume, typically at least 35%. It has been found that the fibers may be three times more thermally conductive than copper. However, the binder is frequently a poor thermal conductor and the fibers themselves are the only significant conduit for the thermal energy. As a result, carbon pitch fiber composite boards have poor thermal conductivity in the direction transverse to the layers of the fibers, and high thermal conductivity through the fibers along the layers.
The problem presented by the poor thermal conductivity transverse to the fibers may be more easily seen with reference to FIGS. 1-3. With reference to the typical installation of FIG. 1, a mounting board 10 may be carried in a rack 20 designed to carry several of the boards and having heat sinks 30 located thereon. When the board is mounted in the rack, heat from the heat source mounted on the mounting surface is conducted through a core 12 to the heat sink 30. Typically, the board is mounted in the rack so that contact is continuously maintained between the core and the sink and, to this end, may be held in the rack with a pressure fitting 32. Because it is important to maintain contact between the ends 14 of the core 12 and the sink 30, a portion 16 of the mounting surface of the core adjacent its ends is generally brought into pressural engagement with the sink 30. Accordingly, and with reference to FIG. 2 where the direction of heat transfer is shown by the arrows, heat from the heat source is transferred from one portion of the mounting surface of the core 12 to another portion of the mounting surface and to the heat sink.
As shown in FIG. 2, the board may have a core 12 formed from a single piece of thermally conductive and isotropic material. The heat can be conducted vertically into the core (direction A in FIG. 2) and the entire cross-sectional area of the core may be used to conduct the heat from the source to the sink. However, the materials of choice for the core 12, aluminum and copper, have significant performance limitations. For example, copper has high thermal conductivity, but also has a high density that limits its use in airborne applications. Aluminum has low density, but also has low thermal conductivity.
When, with reference to FIG. 3, high conductivity fiber composite materials are used in an effort to improve thermal conductivity and reduce weight, the heat is not readily conducted transverse to the layers of the fibers vertically into the core 18 and is generally confined to the mounting surface of the core 18. Because of the reduction in cross-sectional areas of the core effectively conducting heat, the composite board is unable to conduct the heat away from the components as efficiently as the solid boards. Further, the resulting thermal conductivity is significantly less than the array of fibers could potentially achieve.
One known approach to increasing the efficiency of thermal conduction in a composite mounting board is to expose the ends of the fibers in the core to the heat source and the heat sink. With reference to FIG. 4, this may be accomplished by bending the fibers 40 so that their ends reach the mounting surface of the core. The source of heat may be positioned proximate one end 42 of the bent fibers and a heat sink positioned proximate the other end 44 of those fibers. As shown, for example, in U.S. Pat. Nos. 4,867,235 and 4,849,858 to Grapes, et al, it is known to place an insert 46 inside the composite material to help achieve the necessary fiber bending. Bending the fibers and using an insert may, however, introduce complexities to the manufacturing process and may weaken the strength of the board.
A further problem with the bending of the fibers may be more clearly seen with reference to FIG. 5. The ends 42 of the bent fibers form specific area(s) 48 for heat transfer whose location cannot be changed once the board has been manufactured. In order to effect heat transfer, the heat generating components must be placed proximate the specific areas 48. Consequently, boards must be designed and manufactured for each layout of heat generating components. If large numbers of the fibers are bent to create a large heat transfer area in an effort to overcome this problem, the strength of the board may be unacceptably compromised. As a result, a generic composite board has not heretofore been produced that could be adapted to transfer heat from areas throughout the board's lateral surface.
It also is to be understood that in such composite mounting boards, heat from the components may be transferred to only those fibers whose ends reach the mounting surface adjacent the component. Fibers whose ends reach the mounting surface remote from the component do not have heat conducted thereto, although some radiated heat may reach remote fibers ends.
Accordingly, objectives of the present invention are to provide a novel heat transfer device that may be used as a mounting board for heat generating components, a method of transferring heat and a method of manufacturing a heat transfer device that reduce the problems of the prior art and improve the efficiency of heat conduction in composite mounting boards.
It is a further object of the present invention to provide a novel heat transfer device having a core of thermally conductive fibers that are parallel to the mounting surface of the core throughout their length.
It is yet a further object of the present invention to provide a novel heat transfer device that includes heat conducting means for conducting heat into the depth of the core of the device.
It is still a further object of the present invention to provide a novel heat transfer device formed from a composite material having a core of thermally conductive fibers in which heat is conveyed to the interior of the core without bending the fibers.
It is another object of the present invention to provide a novel method of manufacturing a heat transfer device having a core of composite materials in which wedges are placed in cavities in the core for conducting heat to and from fibers in the interior of the core.
It is yet another object of the present invention to provide a novel method of manufacturing a heat transfer device that may be adapted to transfer heat from selected areas on the board's mounting surface after the core of the board has been formed.
It is still another object of the present invention to provide a novel method for transferring heat from a heat source to a heat sink in which heat is conducted through fibers in the interior of a core of composite material.
These and many other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.